For some applications, it is useful to have a single switched mode power converter available that may operate to supply an output voltage that is smaller than an input voltage and at different times to supply an output voltage that is greater than an input voltage. A common application example, illustrated in FIG. 1A, is when an input voltage source 100 such as a battery supplies an input voltage 111 to a power converter 180 which in turn supplies a constant output voltage 113 to an electronic circuit 200.
FIG. 1B illustrates the decay in battery voltage available from a typical Li-ion battery as a function of time. As illustrated in FIG. 1B, when the battery 100 is fully charged, the DC voltage supplied by the battery 100 is larger than the DC voltage required by the circuit 200 (3.3 volts in this example). The intervening power converter 180 therefore operates to “step-down” or “buck” the battery voltage to the voltage required by the circuit 200. A switched mode power supply operating in this manner is often termed a “buck converter”. As the battery 100 discharges, however, the DC voltage supplied by the battery 100 decreases. At a certain point, the DC voltage supplied by the battery 100 is lower than the DC voltage required by the circuit 200. The intervening-power converter 180 then must operate to “step-up” or “boost” the battery voltage to the voltage required by the circuit 200. A switched mode power supply operating in this manner is often termed a “boost converter”. A power supply that is capable of operating to both “buck” and “boost” may be referred to as a “buck-boost” converter. There are many known topologies for buck-boost converters.
For example, the TPS63000 family of buck-boost converters available from Texas Instruments Incorporated of Dallas, Tex. and the LTC3440 family of buck-boost converters available from Linear Technology Corporation of Milpitas, Calif. are representative of a conventional 4-switch topology. The basic topology of a conventional 4-switch power converter may employ a circuit involving an inductor and four switches. A first of the switches (S1) is coupled between an input voltage supply and a first terminal of the inductor. A second of the switches (S2) is coupled between the first terminal of the inductor and a ground node. A third of the switches (S3) is coupled between a second terminal of the inductor and the ground node. A fourth of the switches (S4) is coupled-between the second terminal of the inductor and a load.
In a buck mode, one of four switches is active, another switch is operating as a rectifier, another switch is permanently on, and the last switch is permanently off. In a boost mode, one switch is active, one switch is operating as a rectifier, one switch is permanently on, and one switch is permanently off. The converter automatically switches from step-down operation to step-up operation and back as required. In addition, there may be a four-switch buck-boost operating mode implemented when the input supply voltage is approximately the same as the output voltage. However, regardless of operating mode, there are two switches in series in the output current path-through a power converter of the conventional four-switch topology. Thus, to achieve the same efficiency as a power converter with one switch in the output current path, the switches of a conventional 4-switch topology have half the resistance. In an integrated circuit implementation this results in an increase of die area and cost.
Although the conventional 4-switch topology operates satisfactorily in many applications, improvements are desired.